Linking land use intensity, biodiversity, soil microbial processes and organo-mineral interactions for a mechanistic understanding of full nitrogen balances in grassland ecosystems (BE_BioMON II)

• Applicants: Privatdozent Dr. Michael Dannenmann; Professor Dr. Michael Schloter; Dr. Stefanie Schulz; Dr. Steffen Schweizer
• Subject Area: Soil Sciences ; Ecology of Land Use
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 512359828

In grasslands an increase in land use intensity (LUI) leads to nitrogen (N) losses, a decline of biodiversity and multifunctionality. Within BE_BioMON I our unique 15N tracing experiment demonstrated that grassland fertilization in the year of application is a fertilization of the microbiome and soil organic nitrogen (SON) pools, rather than of plants, which in turn rely on other N sources. This new paradigm strongly increases the importance of gaining more knowledge on longer-term fertilizer N mobilization from different SON pools in the years after fertilization, and on the role of biological nitrogen fixation (BNF), both remaining key unknowns in grassland biogeochemistry. Through an interdisciplinary integration of soil microbial ecology, soil science, stable isotope biogeochemistry and modelling, we aim to fill these knowledge gaps. Our overarching goal is to provide a mechanistic understanding of fertilizer N cycling through different SON pools upon release for N uptake or loss pathways, and to assess the importance of BNF as influenced by LUI and biodiversity. We hypothesize, (1) that the mean residence time of fertilizer-N in SON is a key driver of plant N uptake and ecosystem N balances, with short mean residence times under high LUI leading to mining of SON. (2) We hypothesize that increasing LUI will result in a shift from symbiotic to associative N fixation, which will decrease the N2:N2O emission ratio due to truncated denitrification pathways in symbiotic diazotrophs. (3) We expect that under low LUI, large rates of BNF will significantly contribute to positive N balances, with mulching particularly efficiently re-fueling SON pools via detrital pathways. To test these hypotheses, we will trace the transformations of different 15N fertilizer sources (farmyard manure, liquid slurry) and study the underlying biotic and abiotic processes over five years. With that we assess the role of N storage in and release from ON pools for plant N uptake or N losses in the following years. This will serve to set up fertilizer N balances and to assess the mean residence time of fertilizer-N in different N pools (WP1). Furthermore, we will test to what extent abandoned fertilization in the LUX experiment has affected BNF (WP2.1). This will be complemented by a new mechanistic mesocosm experiment with 15N2 exposure to disentangle LUI effects on the rates and fates of BNF-N as influenced by mowing vs. mulching (WP2.2). The gained knowledge from our cross-phase experimental work and other BExIS data will be used to identify the abiotic and biotic mechanisms driving the changes of full N balances under different LUI (WP3). We expect that this synthesis ultimately allows for an advanced mechanistic and functional understanding of ecosystem N cycling as influenced by LUI, above- and belowground biodiversity, and site-specific properties.

• DFG Programme: Infrastructure Priority Programmes
• Subproject of: SPP 1374: Biodiversity Exploratories
• International Connection: Spain
• Co-Investigator: Privatdozent Dr. Ralf Kiese
• Cooperation Partner: Professor Dr. Eduardo Vazquez Garcia